The problem arisies when you have a mix of retail and non-retail activations.
We have a number of PCs bought when we were a small startup that came with retail versions of Windows. As they are valid licenses (Pro and even Ultimate), we never bothered to re-install the machines. I don't think we are in a unique situation. So a patch that should be applied to some machines and not others depending on what license is installed, without being able to filter them easily in WSUS, is not exactly an ideal situation.